Wireless technology has become an integral part of society with such devices as the pager, cellular phone, personal digital assistant (PDA), as well as networking technology such as wireless routers. With this explosion of wireless technology, there are many instances where a nearby wireless transmitter tends to overwhelm an adjacent receiver. Under these circumstances, it is possible to remove the offending transmit frequency by placing a notch filter at the output of the transmitter and tuning the notch filter to the frequency of the adjacent receiver.
As the RF power level of transmitters increase, it becomes problematic to use conventional notch filters, which reflect the energy at the notch frequency. An example of a commercially available conventional notch filter is the 4DRN40-422.75/X3-O/O notch filter offered by K&L Microwave at 2250 Northwood Drive Salisbury, Md. 21081. FIG. 1 illustrates the measured frequency response of this filter. The lower curve shows the S11 response, which reflects nearly all of the energy at the notch frequency. At the notch frequency, the S11 response rises to approximately −1 dB, which indicates that the reflected energy travels back towards the transmitter. At low RF power levels, this reflected energy can interact with the transmitted energy to create interference signals known as intermodulation distortion products. At high RF power levels, this reflected energy can even physically damage the transmitter.
Notch filters are commonly required to be tunable since it is not always known beforehand which frequencies are required to be removed from a spectrum. Methods of tuning are either mechanical or electrical, and vary significantly in terms of speed. Mechanical tuning mechanisms tend to be slow and can even be manual. Electronic tuning mechanisms are much faster. The fastest method of electronic tuning is to use solid state devices such as PIN diodes, Gallium Arsenide transistors, or other exotic devices. These devices are capable of switching speeds exceeding 1 microsecond and are used when the fastest switching speeds are required.
There have been many published methods for achieving notch filters but none have reported the ability to simultaneously deliver a notch filter that is absorptive with high RF power handling while still being electronically tunable. There have also been publications related to electronically tunable filters that report high RF power handling, but none have reported a means to simultaneously enable low loss, absorptive, high RF power handling, and fast electronic tuning of notch filters.
For example, U.S. Pat. No. 4,694,266, entitled “Notch Filter”, issued to Wright on Sep. 15, 1987, disclosed a notch filter using a quadrature hybrid coupler and two SAW band pass filters. However, the SAW filters are incapable of being tuned electrically, and would reflect energy at the notch frequency, rendering this approach non-absorptive. It also did not disclose high RF power handling capabilities.
U.S. Pat. No. 4,207,547, entitled “Reflection Mode Notch Filter, issued to Buck on Jun. 10, 1980, disclosed quadrature hybrid devices to steer transmitted and reflected energy to provide an absorptive band reject filter. This approach, however, would require a high order band pass filter to implement a high order band reject filter. This approach also used a phase shifter, and therefore is inherently narrow band. Also, because the band pass filter is used in series, the equivalent loss of the notch filter must be at least equal to the band pass filter.
U.S. Pat. No. 7,323,955, entitled “Narrow-band Absorptive Bandstop Filter with Multiple Signal Paths,” issued to Jachowski on Jan. 29, 2008, disclosed notch filtering with absorptive properties using directional couplers to steer signals as well as band pass filters to add and subtract signals to create the notch characteristic. While this approach provided a good absorptive characteristic, it required many additional components in a large circuit. Because it used band pass filters in series, the loss through the band pass filter must be taken into account and precludes the possibility of a low loss notch filter. It also did not disclose high RF power handling capabilities.
U.S. Pat. No. 5,781,084, entitled “Microwave Reflection Filter including a Ladder Network of Resonators Having Progressively Smaller Q Values,” issued to Rhodes on Jul. 14, 1998, described a passive notch filter that is also absorptive. However, this device required the use of a 3 port circulator. Circulators are not low in loss, and more importantly, are not available at UHF frequencies and below, due to size and weight limits. It also did not disclose high RF power handling capabilities.
U.S. Pat. No. 7,174,147, entitled “Bandpass Filter with Tunable Resonator,” issued to Toncich and Fabrega-Sanchez on Feb. 6, 2007, described a method for creating an electronically tunable band pass filter capable of tolerating high RF power levels. This is achieved by using a ferroelectric material as the electronically tunable element. However, ferroelectric materials do not react quickly, thus while this method did allow for electronic tuning, it is limited to applications where fast tuning speed is not a requirement.
A paper published by Swartz et. al., entitled “Large-area Varactor Diode for Electrically Tunable, High-Power UHF Bandpass Filter,” published in November 1980 in the IEEE Transactions on Electron Devices, described a varactor designed for use in electronically tunable high power band pass filters. However, it did not describe a method for achieving this while simultaneously also having an absorptive characteristic.